Resistance element



Patented June 14, 1938 RESISTANCE ELEMENT George M. Cooper, Elkhart, Ind., assignor to Chicago Telephone Supply 00., Elkhart, Ind., a corporation of Indiana No Drawing. Application June 4, 1936, Serial No. 83,495

7 Claims.

My invention relates to resistance elements and more particularly to resistance elements formed by combining conductive particles and synthetic resins.

' Heretofore, resistance elements of this general class have been formed by mixing finely divided conductive particles such as graphite, lampblack,

etc., with resins of the phenol-formaldehyde type and subjecting the mixture to heat to complete 10 the reaction after the mixture was in the desired physical form or. shape.

It has become more and more important, as the radio art has developed, for resistance values to be of greater stability under widely varying conditions of temperature, humidity, wear, and load,

particularly when used in certain circuits. 'Variations that were formerly acceptable can no longer be tolerated due to the greater loads that are being imposed and to the more critical circuit positions in which the resistors are being used.

It has been found that resistance elements composed of conductive particles and a phenol formaldehyde resin are not capable of maintaining their original resistance value over a reasonable period of time, especially if they are subjected to extremes of heat or humidity. The reason for this instability has been traced and found to be caused by the reactions within the resin continuing indefinitely and changing the relationship of the conductive particles within the resistance element. Obviously any resistance element so constituted is not well adapted to use in modern radio receivers.

Resistance elements composed of conductive particles and vinyl resins have also been employed and are satisfactory for low temperature requirements but due' to the vinyl resin becoming plastic at temperatures near 75 C. such resistance elements cannot be used where operating temperatures exceed this value. In many instances it is highly desirable to operate resistance elements above this temperature.

It is, therefore, an object of my invention to provide a resistance element that will maintain its original value within very close limits over a long period of time and under varying conditions of temperature and humidity. as. It is a further object to provide a resistance element thatis economical to manufacture.

50 With the above objects in view and others that will be brought out as the description progresses,

I prefer to accomplish one embodiment of my invention as follows:

I employ as a binding agent an alkyd resin 5 with a suitable modifying agent, Chinese wood Gal oil being very satisfactory, and thoroughly mix with this, suitable conductive particles to provide the desired resistance value. It has been my experience that graphite, lampblack, and carbon black are the most suitable materials for this use. In some cases where very high resistance values are to be obtained I have found it desirable to include a small amount of finely divided inert non-conductive matter such as asbestos which acts as a filler.

I have found that this mixture when carried in a suitable solvent can be applied by spraying or other suitable means, as a relatively thin film to a supporting base of insulating material such as asbestos paper, bakelite or other. suitable material. The film and its supporting base is then placedin a suitable oven and heated, the temperature and length of time being varied for different thicknesses of film and for different percentages of binding agent in the mixture. I find that the resistance film is most stable after it has been subjectedto heat for a period of time sufiicient to render the film insoluble in the solvents which were used before deposition. The heat used in the curing process causes some very complex chemical reactions within the film that appear to convert the binding agent from a soluble state to an insoluble state in which condition it is very resistant to heat, humidity and abrasion. This imparts to the resistance element unusual stability under conditions of high humidity and high temperature or under an electrical load sufficient to cause considerable heating.

I obtain a resistance of approximately 50,000 ohms in a resistance element of ordinary size. with a mixture of the following materials in the indicated proportions:

Per cent Alkyd resin 72 Chinese wood oil 28 Lampblack 14 Solvent for binding agent 61 This mixture is applied to a base of laminated phenolic material by spraying and then placed in an oven at a temperature of approximately 40 C. The oven temperature is gradually increased for one hour to 165 C. and maintained at this point for one more hour. The resistance element is .then withdrawn and cooled after which it is ready for use.

To obtain higher values of resistance the amount of lampblack is reduced and a percentage 01 finely divided inert non-conductive material such as asbestos added. To obtain lower resistance values the percentage oi! alkyd resin and modiiying agent may be reduced or a part of the lampblack replaced with graphite. Where the maximum stability is desired I prefer to use the latter mixture.

The above described process provides a resistance element with a conductive film of approximately .003 inch thickness which has been found to be entirely satisfactory for applications where a supporting base is not objectionable. In cases where such a supporting base is objectionable I can mold a mixture of the above binding agent and lampblack without any solvent by applying heat and pressure to provide a self-supporting resistance element. I find it desirable to increase the ratio of filler and lampblack to the binding agent to keep the resistance value within the desired range and to shorten the molding cycle which is otherwise relatively long. The resistance values obtainable are then controlled by the ratio of filler to lampblack or by the substitution of graphite or carbon black for the lampblack.

I claim:

1. A resistance element composed of conductive material and approximately three parts of an alkyd resin modified with approximately one part'of Chinese wood oil.

2. A resistance element composed of pulverulent carbon and approximately three parts of an alkyd resin modified with approximately one part of Chinese wood oil.

3. A resistance element comprising a supporting base, a thin film of resistance material deposited on said base comprising pulverulent carbon and approximately three parts of an alkyd resin modified with approximately one part of Chinese wood oil.

4. A resistance element composed of conductive material, a binding agent comprising approximately three parts of an alkyd resin and approximately one part of Chinese wood oil, the whole of which has been subjected to heat suflicient to render the binding agent insoluble in the solvents that were effective before said heating.

5. A resistance element comprising a supporting base, a thin film of resistance material deposited on said base, said resistance material comprising a binding material including approximately three parts of an alkyd resin with approximately one part of Chinese wood oil and pulverulent conductive material, the whole of which has been subjected to heat suflicient to render the binding agent insoluble in the solvents that were effective before curing.

6. A resistance element, comprising a binder consisting of approximately 72% alkyd resin,

28% Chinese wood oil, and a finelydivided carbonaceous conducting material.

7. A resistance element, comprising -a binder consisting of approximately 72% alkyd resin, 28% Chinese wood oil, and a finely divided carbonaceous conducting material, the whole of which has been subjected to ,a temperature of not materially less than 165 C. for aperiod of approximately one hour.

GEORGE M. COOPER. 

